Cement set retarding additives, compositions and methods

ABSTRACT

Cement set retarding additives, set retarded hydraulic cement compositions and methods of using the cement compositions for cementing zones in wells are provided. The set retarding additives are copolymers containing monomer units formed from a first monomer of acrylic acid or methacrylic acid and a second monomer of acrylamide or a derivative of acrylamide.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to cement set retardingadditives, hydraulic cement compositions including the additives andmethods of using the set retarded compositions for cementingsubterranean zones in wells.

2. Description of the Prior Art

A variety of procedures involving hydraulic cement compositions areutilized in the construction and repair of wells such as oil, gas andwater wells. For example, in the completion of a well after a well borehas been drilled into one or more subterranean producing formations, apipe such as casing is disposed in the well bore and a hydraulic cementcomposition is pumped into the annular space between the walls of thewell bore and the exterior of the pipe. The cement composition isallowed to set in the annular space whereby an annular cement sheath isformed therein which bonds the pipe to the walls of the well bore andprevents the undesirable flow of fluids into and through the annularspace.

In repairing producing wells, hydraulic cement compositions are oftenutilized to plug holes or cracks in the pipe disposed in the well bore;to plug holes, cracks, voids or channels in the aforementioned cementsheath between the pipe and the well bore; to plug permeable zones orfractures in subterranean formations and the like. The holes, cracks,etc. are repaired by forcing hydraulic cement compositions thereintowhich harden and form impermeable plugs therein.

High temperatures are frequently encountered in deep subterranean zonesto be cemented. The combination of the depth of the zone and the hightemperature thereof often results in the requirement that the settingtime of the cement composition be extended to allow the cementcomposition to be pumped into the zone to be cemented. Set retardingadditives have been developed and used for this purpose, and suchadditives have been mixed with well cement compositions in amountssufficient to delay the setting of the compositions until they can bepumped into desired subterranean locations.

While a variety of set retarding additives have been developed and usedsuccessfully heretofore, they can and often do produce erratic resultsin cement compositions of different component make-up and/or causepremature gelation of cement compositions when they are subjected to thehigh temperature environments of deep subterranean zones. The hightemperatures referred to herein are temperatures of 200° F. and higher,and the gelation referred to is the abnormal increase in viscosity of acement composition without an increase in compressive strength. Theincrease in viscosity makes the cement composition difficult orimpossible to pump even though the cement composition has not set. Inaddition, a number of the prior art set retarding additives arecomprised of materials which are expensive to produce. Thus, there is acontinuing need for improved cement set retarding additives which do notcause premature gelation and which are relatively inexpensive, setretarded hydraulic cement compositions containing the additives andmethods of using the compositions for carrying out cementing operationsin wells.

SUMMARY OF THE INVENTION

Improved set retarding additives, set retarded hydraulic cementcompositions and methods of using the compositions are provided whichmeet the needs described above and overcome the shortcomings of theprior art. The set retarding additives of this invention are comprisedof copolymers containing monomer units formed from a first monomerselected from the group consisting of acrylic acid and methacrylic acidand a second monomer selected from the group consisting of acrylamide,methacrylamide, N,N-dimethylacrylamide, N-methylacrylamide,N-isopropylacrylamide and N-(hydroxymethyl) acrylamide. The monomerunits formed from the first monomer are preferably present in thecopolymers in an amount in the range of from about 10 mole % to about 55mole % of the copolymers with the remainder comprising monomer unitsformed from the second monomer.

Set retarded hydraulic cement compositions including the additives ofthis invention and methods of using the cement compositions in theconstruction and repair of wells are also provided.

It is, therefore, a general object of the present invention to provideimproved cement set retarding additives, set retarded cementcompositions and methods of using the set retarded cement compositionsin well cementing operations.

Other and further features, objects and advantages of the presentinvention will readily suggest themselves to those skilled in the artupon a reading of the description of preferred embodiments whichfollows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The set retarding additives of this invention are relativelyinexpensive, do not cause premature gelation and bring about predictablecement composition thickening and setting times. The additives arecomprised of copolymers containing monomer units formed from a firstmonomer selected from the group consisting of acrylic acid andmethacrylic acid and a second monomer selected from the group consistingof acrylamide, methacrylamide, N,N-dimethylacrylamide,N-methyl-acrylamide, N-isopropylacrylamide and N-(hydroxymethyl)acrylamide. The first monomer is preferably acrylic acid, and the secondmonomer is preferably selected from acrylamide andN,N-dimethylacrylamide. Generally, the set retarding copolymers of thisinvention contain monomer units formed from the first monomer in anamount in the range of from about 10 mole % to about 55 mole % with theremainder of the monomer units being formed from the second monomer.

The most preferred set retarding copolymer is comprised of monomer unitsformed from acrylic acid and monomer units formed fromN,N-dimethylacrylamide, with the monomer units formed from acrylic acidbeing present in the copolymer in an amount in the range of from about40 mole % to about 50 mole % of the copolymer.

The set retarding copolymers of this invention have the followinggeneral structure: ##STR1## wherein: R₁ is selected from hydrogen or amethyl group; R₂ is selected from hydrogen or a methyl group; R₃ isselected from hydrogen, a methyl group, an isopropyl group or ahydroxymethyl group; and R₄ is selected from hydrogen or a methyl group.

When R₁ in the above structural formula is hydrogen, the first monomeris acrylic acid and when R₁ is a methyl group the first monomer ismethacrylic acid. When the second monomer is acrylamide, R₂, R₃ and R₄are all hydrogen. When the second monomer is methacrylamide, R₂ is amethyl group and R₃ and R₄ are each hydrogen. When the second monomer isN,N-dimethylacrylamide, R₂ is hydrogen and R₃ and R₄ are methyl groups.When the second monomer is N-methylacrylamide, R₃ is a methyl group andR₂ and R₄ are hydrogen. When the second monomer isN-isopropyl-acrylamide R3 is an isopropyl group and R₂ and R₄ arehydrogen, and when the second monomer is N-(hydroxymethyl) acrylamide R₃is a hydroxymethyl group (CH₂ OH) and R₂ and R₄ are hydrogen.

In the preparation of the above described set retarding copolymers, themonomers can be polymerized in an aqueous solution using sodiumpersulfate and tetraethylenepentamine as an initiating system. In atypical synthesis, the monomers are dissolved in deionized water and thereaction mixture is maintained in a water bath at 60° C. Nitrogen isbubbled through the reaction mixture to eliminate any dissolved oxygenfor a minimum time of 30 minutes whereupon small quantities of sodiumpersulfate and tetraethylenepentamine are added to the reaction mixture,and the reaction is allowed to proceed for about 4 hours. The aqueoussolution containing the copolymer produced can be utilized directly as aliquid cement composition set retarding additive.

The set retarded hydraulic cement compositions of this invention arebasically comprised of a hydraulic cement, sufficient water to form aslurry of the cement and a copolymer set retarding additive describedabove. While various hydraulic cements can be utilized in the cementcompositions, Portland cement is generally preferred, and can be, forexample, one or more of the various types identified as API Classes A-Hand J cements. These cements are classified and defined in APISpecification for Materials and Testing for Well Cements, APISpecification 10A, 21st Edition dated Sep. 1, 1991, of the AmericanPetroleum Institute, Washington, D.C. API Portland cements generallyhave a maximum particle size of about 90 microns and a specific surface(sometimes referred to as Blaine Fineness) of about 3900 squarecentimeters per gram. A highly useful and effective cement slurry basefor use in accordance with this invention comprises API Class H Portlandcement mixed with water to provide a density of from about 11.3 to about18.0 pounds per gallon.

It is often highly advantageous to use a fine particle size hydrauliccement consisting of particles having diameters no larger than about 30microns and having a Blaine Fineness no less than about 6000 squarecentimeters per gram. Preferably, the fine cement particles havediameters no larger than about 17 microns and most preferably no largerthan about 11 microns. The Blaine Fineness is preferably greater thanabout 7000 square centimeters per gram, more preferably about 10,000square centimeters per gram and still more preferably greater than about13,000 square centimeters per gram. Methods of utilizing such fineparticle size hydraulic cement in well completion and remedialoperations are disclosed in U.S. Pat. No. 5,121,795 issued Jun. 16, 1992and No. 5,125,455 issued Jun. 30, 1992 both of which are incorporatedherein by reference.

The water used in the cement compositions of this invention can be waterfrom any source, provided that it does not contain an excess ofcompounds which adversely react with or otherwise affect othercomponents in the cement compositions. The water is present in a cementcomposition of this invention in an amount sufficient to form a slurryof the cement, preferably, a slurry which is readily pumpable.Generally, the water is present in the range of from about 30% to about60% by weight of dry cement in the composition when the cement is ofnormal particle size. When a cement of fine particle size as describedabove is used, water is present in the cement composition in an amountin the range of from about 100% to about 200% by weight of dry cement inthe composition, and a dispersing agent such as the dispersing agentdescribed in U.S. Pat. No. 4,557,763 issued on Dec. 10, 1985 isgenerally included to facilitate the formation of the cement slurry andprevent the premature gelation thereof.

A copolymer set retarding additive as described above is included in thecement compositions of this invention in amounts sufficient to delay orretard the setting of the compositions for time periods required toplace the compositions in desired locations. When the cementcompositions are utilized to carry out completion, remedial and othercementing operations in subterranean zones penetrated by well bores, thecompositions must remain pumpable for periods of time long enough toplace them in the subterranean zones to be cemented. The thickening andset times of cement compositions are strongly dependent upontemperature, and to obtain optimum results in well applications, aquantity of a copolymer set retarding additive of this inventionrequired to provide the necessary pumping time at the temperature to beencountered is included in a cement composition to be utilized. Suchquantity can be determined in advance by performing thickening timetests of the type described in the above mentioned API Specification10A.

Generally an aqueous solution of a set retarding copolymer of thisinvention which is about 10% active is combined with a cement slurry inan amount whereby the copolymer is present in the resulting set retardedcement composition in an amount in the range of from about 0.1% to about2.0% by weight of dry cement in the composition.

A variety of other additives are often included in well cementcompositions in addition to the set retarding additives. Such otheradditives are well known to those skilled in the art and are included inwell cement compositions to vary the composition density, increase ordecrease strength, control fluid loss, reduce viscosity, increaseresistance to corrosive fluids and the like. Essentially, a cementcomposition meeting the specifications of the American PetroleumInstitute is mixed with water and other additives to provide a cementslurry appropriate for the conditions existing in each individual wellto be cemented.

A particularly preferred set retarded cement composition of thisinvention is comprised of hydraulic cement, water in an amountsufficient to form a slurry of the cement, e.g., an amount in the rangeof from about 30% to about 60% by weight of dry cement in thecomposition, and a copolymer set retarding additive present in thecomposition in an amount in the range of from about 0.1% to about 2.0%by weight of dry cement therein. The set retarding additive is comprisedof a copolymer containing monomer units formed from a first monomerselected from acrylic acid or methacrylic acid and a second monomerselected from acrylamide, methacrylamide, N,N-dimethylacrylamide,N-methylacrylamide, N-isopropylacrylamide and N-(hydroxymethyl)acrylamide.

The methods of this invention for cementing a subterranean zonepenetrated by a well bore are basically comprised of the steps offorming a pumpable set retarded cement composition of this invention,pumping the cement composition into the subterranean zone by way of thewell bore, and then allowing the cement composition to set therein.

In order to further describe the additives, compositions and methods ofthis invention and to facilitate a clear understanding thereof, thefollowing examples are given.

EXAMPLE 1

A first monomer comprised of acrylic acid and a second monomer comprisedof N,N-dimethylacrylamide were copolymerized in an aqueous solutionusing sodium persulfate and tetraethylenepentamine as the initiatingsystem. That is, 8 grams of acrylic acid and 12 grams ofN,N-dimethylacrylamide were dissolved in 200 grams of deionized water.The reaction mixture was placed in a water bath and maintained at 60° C.Nitrogen was bubbled through the reaction mixture to eliminate anydissolved oxygen for about 30 minutes whereupon 0.4 grams of sodiumpersulfate and 0.5 milliliters of tetraethylenepentamine were added tothe reaction mixture. The ensuing reaction was allowed to proceed forabout 4 hours. The resulting reaction product mixture was an aqueoussolution containing about 9.0% by weight of a copolymer comprised ofmonomer units formed from acrylic acid and N,N-dimethylacrylamide. Thecopolymer contained 48.2 mole % monomer units formed from acrylic acidand 51.8 mole % monomer units formed from N,N-dimethylacrylamide.

EXAMPLE 2

Various quantities of the copolymer produced as described in Example 1above (a 9% by weight aqueous solution of the copolymer) were added totest portions of a base cement slurry. The base cement slurry wascomprised of Lone Star Class H hydraulic cement and water in an amountof 38% by weight of dry cement. The base slurry had a density of 16.4pounds per gallon.

The test cement composition samples containing various quantities of theset retarding copolymer were tested for thickening times at varioustemperatures in accordance with the standard API thickening time testingprocedures set forth in the API Specification 10A referred to above. Forcomparison purposes, additional samples of the base cement slurrycontaining various quantities of a commercially available cement setretarding additive were also tested for thickening times at varioustemperatures. The prior art commercially available cement set retardingadditive is comprised of a copolymer of "AMPS®"(2-acrylamido-2-methylpropane sulfonic acid) and acrylic acid and isdescribed in U.S. Pat. No. 5,049,288 issued Sep. 17, 1991 assigned tothe Halliburton Company of Duncan, Okla. The results of these tests areset forth in Table I below.

                  TABLE I                                                         ______________________________________                                        Thickening Times of Set Retarded Cement Compositions                                    Amount of Set Re-                                                   Set Retarding                                                                           tarding Additive in                                                                          Thickening                                           Additive  Cement Composition,                                                                          Times, hr:min                                        in Cement % by Weight of 145°                                                                          200°                                                                        250°                                                                        300°                         Composition                                                                             Dry Cement     F.     F.   F..sup.2                                                                           F..sup.2                            ______________________________________                                        Set Retarder of                                                                         0.2            3:48   4:25 --   --                                  this Invention                                                                Prior Art 0.2            2.53   --   --   --                                  Set Retarder                                                                  Set Retarder of                                                                         0.3            7:23   8:49 --   --                                  this Invention                                                                Prior Art 0.3            5:08   2:55 --   --                                  Set Retarder                                                                  Set Retarder of                                                                         0.5            8:50   9:35 3:10 --                                  this Invention                                                                Prior Art 0.5            --     4:45 --   --                                  Set Retarder                                                                  Set Retarder of                                                                         0.75           --     --   5:10.sup.1                                                                         --                                  this Invention                                                                Prior Art 0.75           --     --   3:10 --                                  Set Retarder                                                                  Set Retarder of                                                                         1.0            --     --   --   3:24.sup.1                          this Invention                                                                ______________________________________                                         .sup.1 Base cement slurry contained water in an amount of 42% by weight o     dry cement.                                                                   .sup. 2 The cement compositions tested above 200° F. included 35%      silica flour by weight of cement.                                        

It can be seen from the results set forth in Table I that the copolymerset retarding additive of this invention produces excellent thickeningtimes at small concentrations at temperatures up to and including 300°F. Further, the copolymer set retarding additive of this invention is astronger retarder than the commercial set retarding additive tested inthat for the same amount of additive, the copolymer of the presentinvention gives longer thickening times. Additionally, the copolymer ofthe present invention can be utilized up to 300° F. whereby thecommercial set retarding additive has an upper temperature limit ofabout 250° F.

EXAMPLE 3

The copolymer set retarding additive prepared as described in Example 1was tested to determine if it causes gelation and was compared with theprior art commercial set retarding additive described in Example 2above.

The two set retarding additives tested were added to samples of the baseslurry described in Example 2 in varying amounts. The gelation testsconsisted of API thickening time tests at 200° F. in which theviscosities of the test cement compositions were determined initiallyand at 25%, 50% and 75% of the total times prior to thickening. Theresults of these tests are given in Table II below.

                                      TABLE II                                    __________________________________________________________________________    Gelation of Set Retarded Cement Compositions                                  Set Retarding Thickening Viscosity                                                                           Viscosity                                                                           Viscosity                                Additive in                                                                          Amount, %                                                                            Time, Initial                                                                            at 25% of                                                                           at 50% of                                                                           at 75% of                                Cement by Weight of                                                                         hr:min                                                                              Viscosity,                                                                         Thickening                                                                          Thickening                                                                          Thickening                               Composition                                                                          Dry Cement                                                                           200° F.                                                                      Bc   Time, Bc                                                                            Time, Bc                                                                            Time, Bc                                 __________________________________________________________________________    Copolymer                                                                            0.3    8:49  20   21    18    --                                       of this                                                                       Invention                                                                     Prior Art                                                                            0.3    2:55   9   30    32    36                                       Retarder                                                                      Copolymer                                                                            0.5    9:35  22   17    16    --                                       of this                                                                       Invention                                                                     Prior Art                                                                            0.5    4:45  14   25    46    52                                       Retarder                                                                      __________________________________________________________________________

From Table II it can be seen that the prior art set retarder causesgelation since the viscosity values increase over the time tothickening. The copolymer set retarding additive of this invention, onthe other hand, does not cause gelation as shown by the viscosityreadings remaining relatively constant during the time prior tothickening. Further, the results set forth in Table II again show thatthe copolymer set retarding additive of this invention is a muchstronger retarder than the tested prior art additive.

EXAMPLE 4

The set retarding additive prepared in accordance with Example 1 wasadded in various quantities to samples of the base cement slurrydescribed in Example 2 above. The resulting cement composition sampleswere tested for 24 hour compressive strengths at 230° F. and 290° F. inaccordance with API Specification 10A. The results of these tests areset forth in Table III below.

                  TABLE III                                                       ______________________________________                                        Compressive Strengths of Set Retarded Cement Compositions                     Amount of Set                                                                 Retarding Additive                                                            in Composition,                                                               % by weight    Compressive Strengths, psi                                     of Dry Cement  230° F.                                                                           290° F.                                      ______________________________________                                        0.3            4800       2280                                                0.5            1657       2260                                                ______________________________________                                    

As can be seen from Table III, the compressive strengths of the cementcompositions of this invention are excellent.

EXAMPLE 5

A copolymer set retarding additive comprised of monomer units formedfrom acrylic acid and acrylamide was prepared. That is, 8 grams ofacrylic acid and 12 grams of acrylamide were dissolved in 200 grams ofdeionized water. The reaction mixture was placed in a water bath andmaintained at 60° C. Nitrogen was bubbled through the reaction mixtureto eliminate any dissolved oxygen for 30 minutes following which 0.2grams of sodium persulfate and 0.25 milliliters oftetraethylenepentamine were added to the reaction mixture. The ensuingreaction was allowed to proceed for about 4 hours. The resultingreaction product mixture was an aqueous solution containing about 10% byweight of a copolymer comprised of 40 mole % acrylic acid monomer unitsand 60 mole % acrylamide monomer units.

The copolymer set retarding additive produced as described above wasadded to a sample of the base cement slurry described in Example 2 abovein an amount of 0.5% by weight of dry cement in the slurry. Theresulting set retarded cement composition was tested for thickening timeat 200° F. in accordance with API Specification 10A. The result of thethickening time test was 6 hours and 14 minutes.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned as well as those which areinherent therein. While numerous changes may be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

What is claimed is:
 1. A method of cementing a subterranean zonepenetrated by a well bore comprising the steps of:forming a pumpable setretarded cement composition comprised of hydraulic cement, sufficientwater to form a pumpable slurry and a set retarding additive comprisinga copolymer consisting essentially of monomer units formed from a firstmonomer selected from the group consisting of acrylic acid andmethacrylic acid and a second monomer selected from the group consistingof acrylamide, methacrylamide, N,N-dimethylacrylamide,N-methyl-acrylamide, N-isopropylacrylamide and N-(hydroxymethyl)acrylamide; pumping said composition into said zone by way of said wellbore; and allowing said cement composition to set.
 2. The method ofclaim 1 wherein said first monomer of said set retarding additive isacrylic acid.
 3. The method of claim 2 wherein said second monomer ofsaid set retarding additive is selected from the group consisting ofacrylamide and N,N-dimethylacrylamide.
 4. The method of claim 1 whereinsaid monomer units of said set retarding additive formed from said firstmonomer are present in said copolymer in an amount in the range of fromabout 10 mole % to about 55 mole %.
 5. The method of claim 1 whereinsaid first monomer of said set retarding additive is acrylic acid, saidsecond monomer is N,N-dimethylacrylamide and said monomer units formedfrom said first monomer are present in said copolymer in an amount inthe range of from about 40 mole % to about 50 mole %.
 6. The method ofclaim 5 wherein said hydraulic cement is Portland cement.
 7. The methodof claim 6 wherein said set retarder is present in said cementcomposition in an amount in the range of from about 0.1% to about 2.0%by weight of cement therein.
 8. The method of claim 19 wherein saidwater is present in said composition in an amount in the range of fromabout 30% to about 60% by weight of cement therein.